Recent studies on the mutagenicity of chemical carcinogens in various systems have shown that there is a positive correlation between mutagenicity and carcinogenicity. Our previous studies with chemical carcinogens in Neurospora crassa not only show a positive correlation, but also indicate that potent chemical carcinogens induce predominantly base-pair substitution mutations. It is necessary to extend our studies to other carcinogenic compounds to determine whether those compounds are also mutagenic in Neurospora and to determine whether those compounds induce base-pair substitution mutations. The ad-3 test system of N. crassa developed by de Serres is used in this study. Conidia are treated with chemical carcinogens. Treated and untreated conidia are assayed for the presence of ad-3 mutants by the direct method. The isolated mutants are analyzed by complementation, dikaryon and trikaryon tests.